American Sign Language: "UNDERSTAND"

Hold your hand up near your forehead.
Your index finger is bent and
the tip of the index finger is touching the pad of your thumb (your
thumbprint). Then flick your index finger so
that it points up.

UNDERSTAND:

Memory aid: Think of a "light bulb" turning on in your head.

DON'T-UNDERSTAND:
Often students ask me "How do you sign 'don't'?" I always have to ask
them what is

the rest of the sentence in which they want to use the concept
of "don't." The reason I need to know the rest of the sentence is because
there isn't just "one" sign for "don't." There are dozens of ways to express
the concept of "don't" in ASL. Most of the time the concept is
"incorporated" into the movement of the sign and the facial expressions or
shaking of the head.
For example, in the sign "don't understand" you express the concept of
"don't" by using a negative headshake: